This invention relates generally to processing a semiconductor wafer to form circuit devices and more specifically it relates to a method for testing a liquid spread on the surface of the wafer to measure the concentration of dopants in the liquid.
In one step of a familiar process for manufacturing circuit devices in a semiconductor wafer, a thin film of a liquid called BPTEOS is deposited on the surface of a wafer. In this abbreviation, the letters B and P stand for components of the liquid which will become impurities or dopants in the wafer. More specifically, P stands for a compound containing a group V element, phosphorous, and B stands for a compound containing a group III element, boron. The preferred components are P2O3 or P2O5 and B2O3.
It would be advantageous to measure the concentration of the B and P components of a film on a relative scale. For example a three part scale corresponds to the common terms normal, low and high and a two part scale corresponds to normal and high (or normal and low).
The terms high and low are defined by their adverse effect on the wafer. For example, a high concentration would cause B and P precipitation during a high temperature treatment of the wafer in a subsequent processing step.
In one known measurement system, a BPTEOS film is irradiated with x-rays and the intensity and wavelength of the radiation that is emitted by the film is measured. The concentrations of the P and B components can be inferred from the intensity and wavelength because they are characteristic of these atoms.
The x-ray technique cannot be used on a production wafer because the energy of the x-ray can damage devices formed on the wafer. The x-ray technique has an additional limitation that it is not very sensitive for the B component.
In another measurement system, the film is irradiated with infra red light and the reflected light is analyzed. The technique is called FTIR from the fact that it produces a Fourier transform of the reflected infra red light. FTIR has low sensitivity for the P component (P2O3).
One object of my invention to provide a new and improved method for non-destructively measuring the concentrations of the B and P components for a production wafer.
I have found that an image histogram for light reflected from a BPTEOS film is characteristic of the relative concentrations of the B and P components.
In a preliminary step, a film is formed on a dummy wafer and the components are measured in this film, using any suitable technique such as the prior art techniques already described. (A destructive test can be used on the test wafer.)
The film and the underlying surface of a test wafer are then illuminated at an array of points and the intensity of reflected light from these points is recorded. The light intensity can be described with a histogram, and an image histogram is a plot of intensity over the range of values. An image histogram has peaks that are characteristic of the concentration of the B and P components of the film. A commercially available machine produces the image histogram.
The test is performed with a normal concentration and with high and/or low concentrations. This data can then be used for production wafers to the extent that the conditions are the same.
Later, a production wafer is tested in the same way and its film is characterized by comparison with the test data.
The histogram also depends on other factors, the thickness of the film and the underlying pattern of the wafer, which I control within suitable limits. As a specific example, I control the film thickness so that its variations are less than 5%.
Other objects and features of the invention will appear in the description of a preferred embodiment.